Skills of Ensemble Hydrologic Forecasts and Potential Use in Guiding Reservoir Operations: Lessons from Central Texas Forecast-Informed Reservoir Operation Pilot

Rapid population growth in Texas has increased the state's vulnerability to extreme weather events, and floods and droughts have exacted a heavy toll on the state’s economy. As Forecast-informed Reservoir Operation (FIRO) is gaining recognition as a strategy to enhance reservoir management nationwide, a consortium was established in Texas to explore the potential application of FIRO in managing a system of reservoirs in central Texas. We will present a comprehensive assessment of skills of ensemble streamflow forecasts from the National Weather Service Hydrologic Ensemble Forecast System (HEFS) at medium and seasonal ranges over the region, with emphasis placed on a) relative skills of reservoir inflow simulations and pool level forecasts from the HEFS with climatology alone and with the integration of medium-range quantitative precipitation forecasts (QPFs), and b) evaluate the ability of the HEFS forecasts to foresee anomalously high and low inflows associated with major flood and drought episodes, key to operational decisions. Key findings include 1) integration of the QPFs enhances the skills of streamflow forecasts for lead times out to 1 month, though the skill gains tend to be small for higher flow thresholds, and the forecasts for low flows are in general less skillful than climatology; 2) the skills of ensemble forecasts for major flooding events tend to be limited as a result of location errors in QPF, inadequate ensemble spread produced by the ensemble forcing processor, and biases in hydrologic model simulations. Efforts being taken to address these limitations are discussed.